This invention provides a fuel tank for automotive applications and other power sources that has improved thermal stability and reduced emissions of fuel vapor.
Concerns about our atmosphere continue to increase along with concerns about US dependence on petroleum products. These factors emphasize developing technologies for automotive and other applications to reduce consumption of motor fuels by conventional internal combustion engines, proposed external combustion engines, combinations of batteries and engines (hybrid vehicles, including plug-in hybrids), and fuel cells. Hybrid vehicles are likely to need supplemental power to achieve commercially acceptable range and power, and petroleum products are likely to continue to be a prominent source of energy for automobiles, trucks, motorcycles, and other consumer equipment.
Modern gasolines contain over one hundred different hydrocarbons with iso-octane as one of the more prominent. Each refiner adds other additives in marketing efforts and to improve combustion efficiency. Additionally the federal government and many state governments require reformulated gasoline that contain oxygenates such as ethanol. At lower concentrations ethanol increases the vapor pressure of gasoline-ethanol blends. Accordingly gasoline tanks contain a complex mix of compounds that tend to produce fuel vapor emissions over a range of temperatures and under a variety of operating conditions.
Fuel vapors are emitted from vehicle fuel tanks when the vehicle is being refueled, and the art provides a fuel vapor recovery canister that is connected to the fuel tank and that contains an adsorbent material for capturing vapors formed during refueling. Conceptually when refueling is finished and the vehicle is driven, air is drawn back through the canister and the resulting air-vapor mixture is ingested into the engine where it is combusted.
The air is expected to regenerate the adsorbent material to enable operation during the next refueling, but complications arise from exhaust emissions caused by the unknown amounts of extra fuel vapor that the engine is ingesting. Further, successfully purging the adsorbent material requires sufficient engine vacuum to draw air through the canister, and boosted engines and direct injected engines occasionally are unable to achieve desired air flow. Purge opportunities for plug-in hybrids can be delayed for several days. Purging also is infrequent for non-automotive applications such as boats.
Adsorption of fuel vapor during refueling is exothermic, and to reduce the size of the canister, U.S. Pat. Nos. Pitel et al. 5,861,050 and Yamazaki et al. 7,309,381 teach the addition of phase change materials into the canister to help cool the adsorbent. These structures increase the need for purging air flow.
While vehicle refueling takes place on an irregular cycle over several days, typical diurnal vehicle operations consist of driving to work, parking the vehicle for up to several hours, and driving the vehicle home, or of driving to and parking for a short time at multiple locations. Daytime temperatures typically add thermal energy to fuel tanks during parking and thereby increase the vapor pressure of the fuel to the point where vapor emissions occur. Vapor emissions also can occur from running losses of automobiles when temperature increases during vehicle operation produce more vapor than the amount of liquid fuel that is being withdrawn from the fuel tank by the engine. Accordingly vehicle emissions of fuel vapors can occur with much greater frequency and in much more material amounts than only during refueling.